1. Field of the Invention
This invention relates to an improved magnetic recording medium and more particularly, it is concerned with a magnetic recording medium with excellent properties in head abrasion, head contamination, drop out, still life, etc.
2. Description of the Prior Art
The magnetic layer of a magnetic recording medium comprises generally a magnetic powder, binder resin, lubricant, abrasive, carbon black and other additives and where high density recording is carried out and a higher durability is required as in the case of a video tape, it meets with a number of problems.
Since the magnetic layer of a video tape is contacted with the video head of a VTR at a high speed and rubbed with each other, much abrasion of the video head occurs. Thus, it is required for a user of it to exchange often the video head, but in general, he cannot exchange so often because of being expensive, unlike phonograph needles. From this point, little head abrasion and high head durability have eagerly been desired. However, if head abrasion is decreased in an easy manner, for example, by decreasing the quantity of an abrasive contained in a magnetic layer or by smoothening the surface of a magnetic layer extremely, the durability of the magnetic layer of a tape lowers, the still life is short and a VTR running system, video head or audio head is contaminated by the magnetic layer thus resulting in fade out of image or drop out of sound. When a control head is contaminated, it is difficult to synchronize and the use of a tape is sometimes impossible.
It is well known that when a video tape is used repeatedly, e.g. 100 times, the opposite side of the base to the magnetic layer, i.e. base surface of the video tape is scraped by a pole in a VTR running system and cassette (in the case of a video cassette) and the scraped base powder adheres to a magnetic layer to cause drop out. It is impossible to overcome completely this disadvantage by the present tape/VTR system. As a method of avoiding substantially this disadvantage on the part of the tape, it is proposed to coat the base surface with a durable protective layer, but this results in an increase in cost and where the surface roughness of the protective layer is large, the surface roughness of the magnetic layer is large (pring through) and the video S/N (monochrome and color) is deteriorated.
On the other hand, a long stil life is an essential condition for a video tape in comparison with audio and computer tapes. As well known, in the reproduction of a video tape of sport match, it is required to stop temporarily and view one scene. "Still life" is used as a measure to represent the durability of a magnetic layer, which is determined by contacting only a part of the tape with a video head under severer conditions such that said contacting should be called "grinding". One effective method for strengthening the still life consists in increasing an abrasive in a magnetic layer, but this increases a head abrasion. Since there is a strong contact between a head and a magnetic layer of a tape, if the magnetic layer is simply made hard, head abrasion is increased and if it is made soft, the strength and still life are lowered and it tends to be scraped to contaminate a video head to such an extent that when a normal tape is reproduced, a picture does not appear sometimes.
For the purpose of preventing head abrasion, addition of a lubricant to a magnetic recording medium has commonly been carried out and it is well known that silicone oils have excellent properties as a lubricant. Above all, high viscosity silicone oils such as dimethylpolysiloxanes with a viscosity of 100,000 centistokes have widely been used because of their properties that the viscosity change is small over a wide temperature range and they scarcely move in the coating. As to application of these high viscosity silicone oils to magnetic tapes, there have been proposed (i) a method comprising incorporating a high viscosity silicone oil with a viscosity of at least 100,000 centistokes in a magnetic layer as disclosed in Japanese Patent Application (OPI) No. 77034/1980, (ii) a method comprising blending a magnetic powder, high viscosity silicone oil, higher aliphatic compound and binder to prepare a magnetic recording medium as disclosed in Japanese Patent Application (OPI) No. 8804/1977 and (iii) a method comprising incorporating a higher fatty acid or a higher fatty acid and silicone oil in a magnetic layer containing a magnetic powder as disclosed in Japanese Patent Application (OPI) No. 84082/1980.
However, these methods meet with the following problems: (i) The tape squeak and running property at a high temperature and high humidity are improved and there are provided excellent audio cassette tapes, but in video tapes requiring a higher durability in higher density recording, there take place an increase of drop out and contaminations of a VTR running system and video head. (ii) The friction coefficient at both a low temperature and a high temperature is low, but the head abrasion, head contamination, increase of drop out and still life are not collectively reviewed. In view of the samples of Examples, these four properties are not excellent, as shown in Comparative Samples. (iii) The metallic tapes using an alloy powder containing iron as a magnetic powder has the drawbacks that the head abrasion and drop out are increased and the still life is short, as shown in Comparative Samples.